Stress alternating amplitude

Figure 8.17 Variation of stress with time that accounts for fatigue failures, (a) Reversed stress cycle, in which the stress alternates from a maximum tensile stress (+) to a maximum compressive stress (-) of equal magnitude, (b) Repeated stress cycle, in which maximum and minimum stresses are asymmetrical relative to the zero-stress level mean stress cr, range of stress cr and stress amplitude cr are indicated, (c) Random stress cycle.

Piezoelectric measurements in bulk samples can also be made by applying an alternating stress and measuring the charge developed. Recent work in this area has been reported by Damjanovic and co-workers using a dynamic press apparatus [17]. In that work, the importance of measurement frequency and stress amplitude was also clearly pointed out. 

When the stress is decomposed into two components the ratio of the in-phase stress to the strain amplitude (j/a, maximum strain) is called the storage modulus. This quantity is labeled G (co) in a shear deformation experiment. The ratio of the out-of-phase stress to the strain amplitude is the loss modulus G"(co). Alternatively, if the strain vector is resolved into its components, the ratio of the in-phase strain to the stress amplitude t is the storage compliance J (m), and the ratio of ihe out-of-phase strain to the stress amplitude is the loss compliance J"(wi). G (co) and J ((x>) are associated with the periodic storage and complete release of energy in the sinusoidal deformation process. Tlie loss parameters G" w) and y"(to) on the other hand reflect the nonrecoverable use of applied mechanical energy to cause flow in the specimen. At a specified frequency and temperature, the dynamic response of a polymer can be summarized by any one of the following pairs of parameters G (x>) and G" (x>), J (vd) and or Ta/yb (the absolute modulus G ) and... 

Several methods, as described in the preceding section, have been used to acquire information relative to the onset of the initial localized plastic deformation under alternating loading. Figure 5 is a plot of reflected light intensity vs. cycles for a transparent PS sample tested at 21 Hz at a stress amplitude of 17.2 MPa The number of cycles, N., to initiate the craze, as determined from the first jump in intensity over background, is about 5,000 cycles and the cycles to fracture, N, is about 11,000. In later sections, the ratio of to Np which in this example is about 0.45, is shown to be a function of both stress amplitude and frequency. 

As was indicated, modification of the CLD is related to modification of the SN-curves. With a modification of the CLD, a modification is meant which does not involve any changes to the experimental SN-curves, which were used to construct the CLD. On the other hand, any SN-curves that are derived from the interpolation can be affected by the CLD-modification. In order to allocate more damage to the small amplitude cycles, some alternative CLDs have been proposed previously [32]. For example, in one alternative CLD the N=1 line is the same as in the linear Goodman diagram, but all other lines are drawn parallel to the N=1 line. In this case, the permissible number of cycles at a certain stress level and mean stress can be found for a lin-log SN-curve using ... 

In a typical fatigue test, a sample is subjected to an alternating stress of a given amplitude and frequency. The cyclic stress amplitude is defined as... 

Fatigue results in a brittle-appearing fracture, with no gross deformation at the fracture. On a macroscopic scale the fracture surface is usually normal to the direction of the principal tensile stress. The crack ejctension depends upon material, amplitudes of alternating stress, frequency of stress, temperature of operation. The process of fatigue failure under a load of a constant amplitude typically includes several stages ... 

The higher the amplitude of alternating stress the smaller the critical crack size. At large Njncub Ngj. With this condition the initiation of a crack is a very... 

---